Apple leafcurling midge
Egg: Eggs are laid singly in the leaf folds or along the margins of developing leaves.
Larva: Larvae are tiny legless maggots, initially pale-yellow but becoming reddish-orange as they develop inside rolled leaves (galls). Full grown they are 1.5-2.5 mm long.
Pupa: Pupae are segmented, light-orange capsules, inside silk cocoons, usually in the soil directly under the infested trees.
Adult: Adults are very small dark (black) flies, 2 or 3 mm in length, with clear wings. Smaller than mosquitoes, they are easily overlooked when sitting on leaves or flying around apple trees during mating or egg laying.
Life History. The biology and developmental timing are not well documented in the Pacific Northwest. Apple leaf midge overwinters as pupae in the soil under infested trees. First generation adults emerge in May, and after mating, the adult females deposit eggs on developing leaves. After larvae complete development inside tightly curled leaves, some may pupate in the rolled leaves but most drop to the ground to pupate in leaf litter or just below the soil surface. Pupae are surrounded by tough, silken cocoons. In the Northwest, two or three generations are produced each season, although suspected overlapping of generations has been reported and timing of generations has been little studied. In Europe, lack of rain is reported to delay larvae from leaving hardened leaves to pupate , which can prolong development time.
Damage. Midge larvae feed along the margins of developing leaves, causing leaves to develop gall-like thickening and tight, inward curling. The infested leaves roll into distorted tubes, may discolor (reddish), become brown and brittle, finally dropping from the tree. Terminal shoots may be stunted as a result, and grafted scions have been reported as seriously retarded, or in some cases, killed.
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